The nutritional requirements of experimental and human normal and malignant cells for sulfur-containing amino acids will be investigated in tissue culture. The effect of an enzyme, capable of degrading sulfur-containing amino acids ("L-methioninase"), upon normal and malignant cells in vitro and in vivo will be studied. Microorganisms producing enzymes similar to L-methioninase, but with different specificities for different sulfur-containing amino acids, will be searched for and extracted.